Within many fields of activity, it is desirable to heat different parts of the body, for example for medical purposes or for reasons of comfort. One example is the heating of joints, hands and other parts of the body of rheumatism sufferers. Another example is within the areas of sports and leisure activities where proposals have been put forward for heated gloves, shoe inlays and other items of clothing as well as sleeping-bags.
It is also desirable in the art to devise containers which, on transport or power failure, maintain an elevated temperature in their storage space. Similarly, there are needs for substrates which, on transport or power failure, maintain an elevated temperature, for example to avoid the risk that the temperature falls in accumulators or batteries placed on the substrates--which could lead to a reduction in the quantity of energy which can be extracted from them.
To cover these needs numerous solutions have been proposed in the art, these all being based on electric heating with the aid of various resistor heating elements and electric energy sources, normally dry cell batteries. As examples of this known technology, mention might be made of Swedish patent application No. 8402743-2 which describes a glove intended for rheumatics with inlaid resistor wires and a current source so as to develop a thermal effect exceeding 25 W in the glove.
In a corresponding manner, Swedish patent application No. 8404783-6 describes a medical undergarment with inlaid resistor heating elements and a current source in the form of a battery. Electrically heated shoe inlays are described, for instance, in the two German specifications DE-A-3 904 603 and DE-C-4 000 259. These shoe inlays have inlaid resistor heating elements which are connectable to a battery or other source of electric energy. In both cases, the resistor heating elements are in the form of printed conductor paths on a base consisting of plastic or other non-conductive material. In both cases, the heating proper is also controlled by means of electric circuit connections so as to as avoid overheating.
As was mentioned above, it is known in the leisure-time sector to make use of electrically heated sleeping-bags or garments. One example is disclosed in US-A-3,443,066 which describes a sleeping-bag or other clothing garment of flexible, thermally insulating material with inlaid resistor heating wires and batteries for electric power supply.
In automotive engineering, similar ideas have been employed, for example to provide heating elements for vehicle seats, an employment which necessitates a degree of give in the inlaid resistor heating elements. One example of this technique is described in SE-B-434 204 (7713250-4), in which the give or extensibility has been achieved by making the inlay of meandering loops of resistor heating wires or tapes in a layer in the padding material of the seat.
Yet a further a example of textile material which can be heated electrically is to be found in US-A-4,845,343 in which the resistor wires have been woven into the fabric structure proper.
All of these prior art devices for heating larger or smaller parts of the body of a patient or general user suffer from numerous common drawbacks. One such drawback is that the prior art resistor heating elements consume considerable quantities of power and are, therefore, expensive in operation. Another drawback is that the possibilities of regulating the heating effect locally, i.e. within a limited part of the body are poor--in other words the user must choose between either a heating effect which gives satisfactory heating of the coldest part of the body which at the same time results in overheating of other parts of the body, or a heating effect which provides comfortable heating of the major part of the body but insufficient heating within certain other parts of the body. A further drawback inherent in many of the prior art devices is that they require separate accumulators whose capacity is often reduced at low temperatures, since they must be carried at relatively unprotected positions in the wearer's clothing. Moreover, the separate accumulators must be carried in some type of belt or the like which may prove unwieldy. Another drawback is that some type of external thermostat is necessary in order to be able to obtain a determined average temperature.